Directional couplers are radio frequency passive devices designed to couple a defined amount of the electromagnetic power in a transmission line to a port enabling the signal to be used in another circuit. Specifically, a directional coupler is designed to couple power flowing in one direction. Typically, a directional coupler is structured using a pair of coupled transmission lines. The transmission lines can be realized using coaxial and the planar technologies. Directional couplers manufactured using planar technologies include strip-lines or micro-strips as the transmission lines and are known as “miniature directional couplers”.
FIG. 1 is a schematic diagram of a conventional directional coupler 100. The coupler 100 includes a pair of transmission lines 110 and 120. The transmission line 110 is the main transmission line, while the second line 120 is the coupling transmission line. A first end and a second end of the main transmission line 110 are an input port 111 and an output port 112, respectively. A first end and a second end of the coupling transmission line 120 are a coupled port 121 and an isolation port 122, respectively.
The transmission lines 110 and 120 are placed (fabricated) in close proximity to each other such that the main transmission line 110 electromagnetically couples with the coupling transmission line 120. Such coupling causes a mutual inductance and a mutual capacitance between the transmission lines 110 and 120.
A radio frequency (RF) signal is provided at the input port 111. Due to the mutual inductance and the mutual capacitance between the main transmission line 110 and the coupling transmission line 120, a portion of the input RF signal is induced in the coupling transmission line 120. The induced RF signal traverses through the coupling transmission line 120 and is output at the coupled port 121. The remaining portion of the input RF signal traverses through the main transmission line 110 and is obtained from the output port 112.
An important property of a directional coupler is a coupling factor (CF), which is the ratio of the power of the induced signal at the coupling port 121 to the power of the input signal at the input port 111. The coupling factor is measured in decibels (dB). The value of the coupling factor depends on the frequency of an input RF signal, the dimensional tolerances of the spacing of the transmission lines 110 and 120 (i.e., the closer the lines are to each other, the higher the CF is), and the length of the transmission lines 110 and 120 (i.e., the longer the lines are, the higher the CF is).
Specifically, the CF value of a directional coupler is not an absolute function of the transmission lines' lengths, but is a function of the ratio of the length to a wave length of the input RF signal. This phenomenon causes a strong dependence of the coupling factor on the frequency of the input (propagating) RF signal. As demonstrated in FIG. 2, the coupling factor value (labeled as 210) changes or increases as a frequency of the input RF signal increases.
However, for proper application of directional couplers, the value of the coupling factor should be substantially constant for an operating frequency band. The strong dependence of the CF on frequency may therefore limit the possible bandwidth of the couple. For example, a substantially constant would be considered typical allowed tolerance for CF variation is about +/−1 dB, which limits the bandwidth of conventional directional couplers to about 300 MHz.
The narrow bandwidth is a limiting factor for some applications (e.g., when directional couplers are integrated in some circuits). For example, the bandwidths of a directional coupler integrated in a power control circuit installed in a cellular telephone would operate in the entire frequency band of RF signals transmitted by the cellular telephone. For example, in modern communication standards, the frequency band of cellular telephones is between 3 GHz and 4 GHz. As demonstrated above, conventional directional couplers cannot meet this demand.
It would therefore be advantageous to provide a solution that would overcome the challenges noted above.